(function () {
  // 将 this 缓存到 root 变量
  var root = this;

  // 防冲突，将被 underscore 覆盖的 _ 保存下来
  var previousUnderscore = root._;

  // 缓存 Array, Object, Function 的 prototype
  var ArrayProto = Array.prototype,
    ObjProto = Object.prototype,
    FuncProto = Function.prototype;

  // 缓存数组和对象常用方法
  var push = ArrayProto.push,
    slice = ArrayProto.slice,
    toString = ObjProto.toString,
    hasOwnProperty = ObjProto.hasOwnProperty;

  // 缓存数组和对象的常用方法
  var nativeIsArray = Array.isArray,
    nativeKeys = Object.keys,
    nativeBind = FuncProto.bind,
    nativeCreate = Object.create;

  // 空函数，创建对象时使用
  var Ctor = function () {};

  // 用于挂载的 _ 既是一个函数，也是一个对象
  // 如果不用 new 调用，就返回一个 new 对象
  var _ = function (obj) {
    if (obj instanceof _) return obj;
    if (!(this instanceof _)) return new _(obj);
    this._wrapped = obj;
  };

  // 将 _ 挂载到 this 上，在 node 中作为模块导出
  if (typeof exports !== "undefined") {
    if (typeof module !== "undefined" && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root._ = _;
  }

  // 设置版本号
  _.VERSION = "1.8.3";

  // Internal function that returns an efficient (for current engines) version
  // of the passed-in callback, to be repeatedly applied in other Underscore
  // functions.

  // 优化 callback 方法，根据 argCount 返回拥有不同个数参数的 callback
  var optimizeCb = function (func, context, argCount) {
    if (context === void 0) return func;
    switch (argCount == null ? 3 : argCount) {
      case 1:
        return function (value) {
          return func.call(context, value);
        };
      case 2:
        return function (value, other) {
          return func.call(context, value, other);
        };
      case 3:
        return function (value, index, collection) {
          return func.call(context, value, index, collection);
        };
      case 4:
        return function (accumulator, value, index, collection) {
          return func.call(context, accumulator, value, index, collection);
        };
    }
    return function () {
      return func.apply(context, arguments);
    };
  };

  // A mostly-internal function to generate callbacks that can be applied
  // to each element in a collection, returning the desired result — either
  // identity, an arbitrary callback, a property matcher, or a property accessor.

  // 内置函数，根据入参 value，创建不同的 callback，用于根据 collection 中每个元素的处理
  var cb = function (value, context, argCount) {
    if (value == null) return _.identity;
    if (_.isFunction(value)) return optimizeCb(value, context, argCount);
    if (_.isObject(value)) return _.matcher(value);
    return _.property(value);
  };

  // 迭代函数，使用 cb 生成
  _.iteratee = function (value, context) {
    return cb(value, context, Infinity);
  };

  // An internal function for creating assigner functions.
  // assign 工厂函数，根据条件创建一个 assign 合并对象
  // 重点在于如何获取到 source 的 key => keysFunc
  var createAssigner = function (keysFunc, undefinedOnly) {
    return function (obj) {
      var length = arguments.length;
      if (length < 2 || obj == null) return obj;
      for (var index = 1; index < length; index++) {
        var source = arguments[index],
          keys = keysFunc(source),
          l = keys.length;
        for (var i = 0; i < l; i++) {
          var key = keys[i];
          if (!undefinedOnly || obj[key] === void 0) obj[key] = source[key];
        }
      }
      return obj;
    };
  };

  // An internal function for creating a new object that inherits from another.
  // 创建对象，Object.create() 的垫片
  var baseCreate = function (prototype) {
    if (!_.isObject(prototype)) return {};
    if (nativeCreate) return nativeCreate(prototype);
    Ctor.prototype = prototype;
    var result = new Ctor();
    Ctor.prototype = null;
    return result;
  };

  // 内部函数，返回一个绑定key的函数，获取key对应的值
  var property = function (key) {
    return function (obj) {
      return obj == null ? void 0 : obj[key];
    };
  };

  // Helper for collection methods to determine whether a collection
  // should be iterated as an array or as an object
  // Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
  // Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
  var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
  var getLength = property("length");

  // 判断是否是数组或类数组
  var isArrayLike = function (collection) {
    var length = getLength(collection);
    return (
      typeof length == "number" && length >= 0 && length <= MAX_ARRAY_INDEX
    );
  };

  // Collection Functions
  // --------------------

  // The cornerstone, an `each` implementation, aka `forEach`.
  // Handles raw objects in addition to array-likes. Treats all
  // sparse array-likes as if they were dense.
  // 遍历数组或者对象，并应用优化过的 iteratee，优化过的 iteratee接收三个参数
  // 注意它会更改原来的obj
  _.each = _.forEach = function (obj, iteratee, context) {
    iteratee = optimizeCb(iteratee, context);
    var i, length;
    if (isArrayLike(obj)) {
      for (i = 0, length = obj.length; i < length; i++) {
        iteratee(obj[i], i, obj);
      }
    } else {
      var keys = _.keys(obj);
      for (i = 0, length = keys.length; i < length; i++) {
        iteratee(obj[keys[i]], keys[i], obj);
      }
    }
    return obj;
  };

  // Return the results of applying the iteratee to each element.
  // 数组和对象的映射
  _.map = _.collect = function (obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    var keys = !isArrayLike(obj) && _.keys(obj),
      length = (keys || obj).length,
      results = Array(length);
    for (var index = 0; index < length; index++) {
      var currentKey = keys ? keys[index] : index;
      results[index] = iteratee(obj[currentKey], currentKey, obj);
    }
    return results;
  };

  // Create a reducing function iterating left or right.
  // 创建 reducer 工厂函数
  function createReduce(dir) {
    // Optimized iterator function as using arguments.length
    // in the main function will deoptimize the, see #1991.
    function iterator(obj, iteratee, memo, keys, index, length) {
      for (; index >= 0 && index < length; index += dir) {
        var currentKey = keys ? keys[index] : index;
        memo = iteratee(memo, obj[currentKey], currentKey, obj);
      }
      return memo;
    }

    return function (obj, iteratee, memo, context) {
      iteratee = optimizeCb(iteratee, context, 4);
      var keys = !isArrayLike(obj) && _.keys(obj),
        length = (keys || obj).length,
        index = dir > 0 ? 0 : length - 1;
      // Determine the initial value if none is provided.
      if (arguments.length < 3) {
        memo = obj[keys ? keys[index] : index];
        index += dir;
      }
      return iterator(obj, iteratee, memo, keys, index, length);
    };
  }

  // **Reduce** builds up a single result from a list of values, aka `inject`,
  // or `foldl`.
  _.reduce = _.foldl = _.inject = createReduce(1);

  // The right-associative version of reduce, also known as `foldr`.
  _.reduceRight = _.foldr = createReduce(-1);

  // 既可用于 arrayLike 也可用于 object，先找到 key，最后返回 value
  _.find = _.detect = function (obj, predicate, context) {
    var key;
    if (isArrayLike(obj)) {
      key = _.findIndex(obj, predicate, context);
    } else {
      key = _.findKey(obj, predicate, context);
    }
    if (key !== void 0 && key !== -1) return obj[key];
  };

  // 过滤器，通过 predicate 真值判断函数循环过滤
  _.filter = _.select = function (obj, predicate, context) {
    var results = [];
    predicate = cb(predicate, context);
    _.each(obj, function (value, index, list) {
      if (predicate(value, index, list)) results.push(value);
    });
    return results;
  };

  // 找出不符合 predicate 的结果
  _.reject = function (obj, predicate, context) {
    return _.filter(obj, _.negate(cb(predicate)), context);
  };

  // Determine whether all of the elements match a truth test.
  // Aliased as `all`.
  // 判断 collection 里每个元素是否都符合 predicate，如果都符合返回 true，否则返回 false
  _.every = _.all = function (obj, predicate, context) {
    predicate = cb(predicate, context);
    var keys = !isArrayLike(obj) && _.keys(obj),
      length = (keys || obj).length;
    for (var index = 0; index < length; index++) {
      var currentKey = keys ? keys[index] : index;
      if (!predicate(obj[currentKey], currentKey, obj)) return false;
    }
    return true;
  };

  // 判断 collection 中的元素是否有符合 predicate 的，如果有则返回 true, 否则 false
  _.some = _.any = function (obj, predicate, context) {
    predicate = cb(predicate, context);
    var keys = !isArrayLike(obj) && _.keys(obj),
      length = (keys || obj).length;
    for (var index = 0; index < length; index++) {
      var currentKey = keys ? keys[index] : index;
      if (predicate(obj[currentKey], currentKey, obj)) return true;
    }
    return false;
  };

  // Determine if the array or object contains a given item (using `===`).
  // Aliased as `includes` and `include`.
  // 数组或者对象是否包含某个 item
  _.contains = _.includes = _.include = function (obj, item, fromIndex, guard) {
    if (!isArrayLike(obj)) obj = _.values(obj);
    if (typeof fromIndex != "number" || guard) fromIndex = 0;
    return _.indexOf(obj, item, fromIndex) >= 0;
  };

  // Invoke a method (with arguments) on every item in a collection.
  // 在 obj 的每个 value 上触发一个方法
  _.invoke = function (obj, method) {
    var args = slice.call(arguments, 2);
    var isFunc = _.isFunction(method);
    return _.map(obj, function (value) {
      var func = isFunc ? method : value[method];
      return func == null ? func : func.apply(value, args);
    });
  };

  // Convenience version of a common use case of `map`: fetching a property.
  // 获取 collection 每个 element 的 key 对应 value
  _.pluck = function (obj, key) {
    return _.map(obj, _.property(key));
  };

  // Convenience version of a common use case of `filter`: selecting only objects
  // containing specific `key:value` pairs.
  // 获取集合中所有包含指定 key:value 的对象
  _.where = function (obj, attrs) {
    return _.filter(obj, _.matcher(attrs));
  };

  // Convenience version of a common use case of `find`: getting the first object
  // containing specific `key:value` pairs.
  // 获取集合中包含指定 key:value 的第一个对象
  _.findWhere = function (obj, attrs) {
    return _.find(obj, _.matcher(attrs));
  };

  // Return the maximum element (or element-based computation).
  // 找最大值
  _.max = function (obj, iteratee, context) {
    var result = -Infinity,
      lastComputed = -Infinity,
      value,
      computed;
    if (iteratee == null && obj != null) {
      obj = isArrayLike(obj) ? obj : _.values(obj);
      for (var i = 0, length = obj.length; i < length; i++) {
        value = obj[i];
        if (value > result) {
          result = value;
        }
      }
    } else {
      iteratee = cb(iteratee, context);
      _.each(obj, function (value, index, list) {
        computed = iteratee(value, index, list);
        if (
          computed > lastComputed ||
          (computed === -Infinity && result === -Infinity)
        ) {
          result = value;
          lastComputed = computed;
        }
      });
    }
    return result;
  };

  // Return the minimum element (or element-based computation).
  // 找最小值
  _.min = function (obj, iteratee, context) {
    var result = Infinity,
      lastComputed = Infinity,
      value,
      computed;
    if (iteratee == null && obj != null) {
      obj = isArrayLike(obj) ? obj : _.values(obj);
      for (var i = 0, length = obj.length; i < length; i++) {
        value = obj[i];
        if (value < result) {
          result = value;
        }
      }
    } else {
      iteratee = cb(iteratee, context);
      _.each(obj, function (value, index, list) {
        computed = iteratee(value, index, list);
        if (
          computed < lastComputed ||
          (computed === Infinity && result === Infinity)
        ) {
          result = value;
          lastComputed = computed;
        }
      });
    }
    return result;
  };

  // Shuffle a collection, using the modern version of the
  // [Fisher-Yates shuffle](http://en.wikipedia.org/wiki/Fisher–Yates_shuffle).
  // 洗牌算法，文章补充：https://juejin.im/post/6844903778315943949   https://segmentfault.com/a/1190000009485000
  // https://github.com/classicemi/blog/issues/6
  _.shuffle = function (obj) {
    var set = isArrayLike(obj) ? obj : _.values(obj);
    var length = set.length;
    var shuffled = Array(length);
    for (var index = 0, rand; index < length; index++) {
      rand = _.random(0, index);
      if (rand !== index) shuffled[index] = shuffled[rand];
      shuffled[rand] = set[index];
    }
    return shuffled;
  };

  // Sample **n** random values from a collection.
  // If **n** is not specified, returns a single random element.
  // The internal `guard` argument allows it to work with `map`.
  // 随机抽取 n 个
  _.sample = function (obj, n, guard) {
    if (n == null || guard) {
      if (!isArrayLike(obj)) obj = _.values(obj);
      return obj[_.random(obj.length - 1)];
    }
    return _.shuffle(obj).slice(0, Math.max(0, n));
  };

  // Sort the object's values by a criterion produced by an iteratee.
  // 根据指定 iteratee 进行排序
  _.sortBy = function (obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    return _.pluck(
      _.map(obj, function (value, index, list) {
        return {
          value: value,
          index: index,
          criteria: iteratee(value, index, list),
        };
      }).sort(function (left, right) {
        var a = left.criteria;
        var b = right.criteria;
        if (a !== b) {
          if (a > b || a === void 0) return 1;
          if (a < b || b === void 0) return -1;
        }
        return left.index - right.index;
      }),
      "value"
    );
  };

  // An internal function used for aggregate "group by" operations.
  // 分组内部函数，用于生成 groupBy 等
  var group = function (behavior) {
    return function (obj, iteratee, context) {
      var result = {};
      iteratee = cb(iteratee, context);
      _.each(obj, function (value, index) {
        var key = iteratee(value, index, obj);
        behavior(result, value, key);
      });
      return result;
    };
  };

  // Groups the object's values by a criterion. Pass either a string attribute
  // to group by, or a function that returns the criterion.
  _.groupBy = group(function (result, value, key) {
    if (_.has(result, key)) result[key].push(value);
    else result[key] = [value];
  });

  // Indexes the object's values by a criterion, similar to `groupBy`, but for
  // when you know that your index values will be unique.
  _.indexBy = group(function (result, value, key) {
    result[key] = value;
  });

  // Counts instances of an object that group by a certain criterion. Pass
  // either a string attribute to count by, or a function that returns the
  // criterion.
  // 统计个数
  _.countBy = group(function (result, value, key) {
    if (_.has(result, key)) result[key]++;
    else result[key] = 1;
  });

  // Safely create a real, live array from anything iterable.
  // 将数组，类数组，对象转化成真正的数组
  _.toArray = function (obj) {
    if (!obj) return [];
    if (_.isArray(obj)) return slice.call(obj);
    if (isArrayLike(obj)) return _.map(obj, _.identity);
    return _.values(obj);
  };

  // Return the number of elements in an object.
  // 获取数组或者对象的 size，对象就看有多少 key:value
  _.size = function (obj) {
    if (obj == null) return 0;
    return isArrayLike(obj) ? obj.length : _.keys(obj).length;
  };

  // Split a collection into two arrays: one whose elements all satisfy the given
  // predicate, and one whose elements all do not satisfy the predicate.
  // 将一个 collection 的元素按照 predicate 结果分成两部分
  _.partition = function (obj, predicate, context) {
    predicate = cb(predicate, context);
    var pass = [],
      fail = [];
    _.each(obj, function (value, key, obj) {
      (predicate(value, key, obj) ? pass : fail).push(value);
    });
    return [pass, fail];
  };

  // Array Functions
  // ---------------

  // Get the first element of an array. Passing **n** will return the first N
  // values in the array. Aliased as `head` and `take`. The **guard** check
  // allows it to work with `_.map`.
  // 返回前 n 个元素
  _.first = _.head = _.take = function (array, n, guard) {
    if (array == null) return void 0;
    if (n == null || guard) return array[0];
    return _.initial(array, array.length - n);
  };

  // Returns everything but the last entry of the array. Especially useful on
  // the arguments object. Passing **n** will return all the values in
  // the array, excluding the last N.
  // 返回 除了最后 n 个元素
  _.initial = function (array, n, guard) {
    return slice.call(
      array,
      0,
      Math.max(0, array.length - (n == null || guard ? 1 : n))
    );
  };

  // Get the last element of an array. Passing **n** will return the last N
  // values in the array.
  _.last = function (array, n, guard) {
    if (array == null) return void 0;
    if (n == null || guard) return array[array.length - 1];
    return _.rest(array, Math.max(0, array.length - n));
  };

  // Returns everything but the first entry of the array. Aliased as `tail` and `drop`.
  // Especially useful on the arguments object. Passing an **n** will return
  // the rest N values in the array.
  // 返回除了前 n 个剩下的
  _.rest = _.tail = _.drop = function (array, n, guard) {
    return slice.call(array, n == null || guard ? 1 : n);
  };

  // Trim out all falsy values from an array.
  // 去掉 null undefined 等这些假值
  _.compact = function (array) {
    return _.filter(array, _.identity);
  };

  // Internal implementation of a recursive `flatten` function.
  // 内部函数，数组扁平化，shadow 为 true 就只扁平化一层
  var flatten = function (input, shallow, strict, startIndex) {
    var output = [],
      idx = 0;
    for (var i = startIndex || 0, length = getLength(input); i < length; i++) {
      var value = input[i];
      if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {
        //flatten current level of array or arguments object
        if (!shallow) value = flatten(value, shallow, strict);
        var j = 0,
          len = value.length;
        output.length += len;
        while (j < len) {
          output[idx++] = value[j++];
        }
      } else if (!strict) {
        output[idx++] = value;
      }
    }
    return output;
  };

  // Flatten out an array, either recursively (by default), or just one level.
  // 根据 flatten 生成
  _.flatten = function (array, shallow) {
    return flatten(array, shallow, false);
  };

  // Return a version of the array that does not contain the specified value(s).
  // 取 array 中不是后面参数的值 _.without([1,2,3], 2,3)
  _.without = function (array) {
    return _.difference(array, slice.call(arguments, 1));
  };

  // Produce a duplicate-free version of the array. If the array has already
  // been sorted, you have the option of using a faster algorithm.
  // Aliased as `unique`.
  // 数组去重，isSorted为是否排序，如果排过序，去重效率大增
  _.uniq = _.unique = function (array, isSorted, iteratee, context) {
    if (!_.isBoolean(isSorted)) {
      context = iteratee;
      iteratee = isSorted;
      isSorted = false;
    }
    if (iteratee != null) iteratee = cb(iteratee, context);
    var result = [];
    var seen = [];
    for (var i = 0, length = getLength(array); i < length; i++) {
      var value = array[i],
        computed = iteratee ? iteratee(value, i, array) : value;
      if (isSorted) {
        if (!i || seen !== computed) result.push(value);
        seen = computed;
      } else if (iteratee) {
        if (!_.contains(seen, computed)) {
          seen.push(computed);
          result.push(value);
        }
      } else if (!_.contains(result, value)) {
        result.push(value);
      }
    }
    return result;
  };

  // Produce an array that contains the union: each distinct element from all of
  // the passed-in arrays.
  // 扁平化并去重
  _.union = function () {
    return _.uniq(flatten(arguments, true, true));
  };

  // Produce an array that contains every item shared between all the
  // passed-in arrays.
  // 取多个数组的交集，判断逻辑：遍历第一个数组的ele，再遍历剩余数组组成的array，如果剩余数组中item都有ele，就push
  _.intersection = function (array) {
    var result = [];
    var argsLength = arguments.length;
    for (var i = 0, length = getLength(array); i < length; i++) {
      var item = array[i];
      if (_.contains(result, item)) continue;
      for (var j = 1; j < argsLength; j++) {
        if (!_.contains(arguments[j], item)) break;
      }
      if (j === argsLength) result.push(item);
    }
    return result;
  };

  // Take the difference between one array and a number of other arrays.
  // Only the elements present in just the first array will remain.
  // 取差集，即参数 array 中有，而 rest 中没有的
  _.difference = function (array) {
    var rest = flatten(arguments, true, true, 1);
    return _.filter(array, function (value) {
      return !_.contains(rest, value);
    });
  };

  // Zip together multiple lists into a single array -- elements that share
  // an index go together.
  _.zip = function () {
    return _.unzip(arguments);
  };

  // Complement of _.zip. Unzip accepts an array of arrays and groups
  // each array's elements on shared indices
  // unzip
  _.unzip = function (array) {
    var length = (array && _.max(array, getLength).length) || 0;
    var result = Array(length);

    for (var index = 0; index < length; index++) {
      result[index] = _.pluck(array, index);
    }
    return result;
  };

  // Converts lists into objects. Pass either a single array of `[key, value]`
  // pairs, or two parallel arrays of the same length -- one of keys, and one of
  // the corresponding values.
  // 将 [[key,value], [key,value]...]转化为 {} 形式
  _.object = function (list, values) {
    var result = {};
    for (var i = 0, length = getLength(list); i < length; i++) {
      if (values) {
        result[list[i]] = values[i];
      } else {
        result[list[i][0]] = list[i][1];
      }
    }
    return result;
  };

  // Generator function to create the findIndex and findLastIndex functions
  // 生成一个找索引的工厂函数
  // 根据 dir，如果为1，从左向右遍历，-1从右向左遍历，找到了就返回相应索引，没找到就返回-1
  function createPredicateIndexFinder(dir) {
    return function (array, predicate, context) {
      predicate = cb(predicate, context);
      var length = getLength(array);
      var index = dir > 0 ? 0 : length - 1;
      for (; index >= 0 && index < length; index += dir) {
        if (predicate(array[index], index, array)) return index;
      }
      return -1;
    };
  }

  // Returns the first index on an array-like that passes a predicate test
  _.findIndex = createPredicateIndexFinder(1);
  _.findLastIndex = createPredicateIndexFinder(-1);

  // Use a comparator function to figure out the smallest index at which
  // an object should be inserted so as to maintain order. Uses binary search.
  // 二分查找将 obj 放到数组里哪个位置，数组默认是有序的
  _.sortedIndex = function (array, obj, iteratee, context) {
    iteratee = cb(iteratee, context, 1);
    var value = iteratee(obj);
    var low = 0,
      high = getLength(array);
    while (low < high) {
      var mid = Math.floor((low + high) / 2);
      if (iteratee(array[mid]) < value) low = mid + 1;
      else high = mid;
    }
    return low;
  };

  // Generator function to create the indexOf and lastIndexOf functions
  // 内部工厂函数，用于生成 index 查找器
  function createIndexFinder(dir, predicateFind, sortedIndex) {
    return function (array, item, idx) {
      var i = 0,
        length = getLength(array);
      if (typeof idx == "number") {
        if (dir > 0) {
          i = idx >= 0 ? idx : Math.max(idx + length, i);
        } else {
          length = idx >= 0 ? Math.min(idx + 1, length) : idx + length + 1;
        }
      } else if (sortedIndex && idx && length) {
        idx = sortedIndex(array, item);
        return array[idx] === item ? idx : -1;
      }

      if (item !== item) {
        idx = predicateFind(slice.call(array, i, length), _.isNaN);
        return idx >= 0 ? idx + i : -1;
      }

      for (
        idx = dir > 0 ? i : length - 1;
        idx >= 0 && idx < length;
        idx += dir
      ) {
        if (array[idx] === item) return idx;
      }
      return -1;
    };
  }

  // Return the position of the first occurrence of an item in an array,
  // or -1 if the item is not included in the array.
  // If the array is large and already in sort order, pass `true`
  // for **isSorted** to use binary search.
  _.indexOf = createIndexFinder(1, _.findIndex, _.sortedIndex);
  _.lastIndexOf = createIndexFinder(-1, _.findLastIndex);

  // Generate an integer Array containing an arithmetic progression. A port of
  // the native Python `range()` function. See
  // [the Python documentation](http://docs.python.org/library/functions.html#range).
  // 生成相应数组
  _.range = function (start, stop, step) {
    if (stop == null) {
      stop = start || 0;
      start = 0;
    }
    step = step || 1;

    var length = Math.max(Math.ceil((stop - start) / step), 0);
    var range = Array(length);

    for (var idx = 0; idx < length; idx++, start += step) {
      range[idx] = start;
    }

    return range;
  };

  // Function (ahem) Functions
  // ------------------

  // Determines whether to execute a function as a constructor
  // or a normal function with the provided arguments
  // 绑定context方法，根据调用方式（直接调用和new）来绑定 this
  /*
  @param sourceFunc 待绑定的函数
  @param boundFunc 绑定完的函数
  @param context 要绑定的this
  @param callingContext 调用时函数里的 this
  @param args 参数
   */
  var executeBound = function (
    sourceFunc,
    boundFunc,
    context,
    callingContext,
    args
  ) {
    if (!(callingContext instanceof boundFunc))
      return sourceFunc.apply(context, args);
    var self = baseCreate(sourceFunc.prototype);
    var result = sourceFunc.apply(self, args);
    if (_.isObject(result)) return result;
    return self;
  };

  // Create a function bound to a given object (assigning `this`, and arguments,
  // optionally). Delegates to **ECMAScript 5**'s native `Function.bind` if
  // available.
  // 绑定 this
  _.bind = function (func, context) {
    if (nativeBind && func.bind === nativeBind)
      return nativeBind.apply(func, slice.call(arguments, 1));
    if (!_.isFunction(func))
      throw new TypeError("Bind must be called on a function");
    var args = slice.call(arguments, 2);
    var bound = function () {
      return executeBound(
        func,
        bound,
        context,
        this,
        args.concat(slice.call(arguments))
      );
    };
    return bound;
  };

  // Partially apply a function by creating a version that has had some of its
  // arguments pre-filled, without changing its dynamic `this` context. _ acts
  // as a placeholder, allowing any combination of arguments to be pre-filled.
  // 偏函数，支持占位符，点位符默认是_
  _.partial = function (func) {
    var boundArgs = slice.call(arguments, 1);
    var bound = function () {
      var position = 0,
        length = boundArgs.length;
      var args = Array(length);
      for (var i = 0; i < length; i++) {
        args[i] = boundArgs[i] === _ ? arguments[position++] : boundArgs[i];
      }
      while (position < arguments.length) args.push(arguments[position++]);
      return executeBound(func, bound, this, this, args);
    };
    return bound;
  };

  // Bind a number of an object's methods to that object. Remaining arguments
  // are the method names to be bound. Useful for ensuring that all callbacks
  // defined on an object beloeg to it.
  // 将对象的方法this全部绑定为对象本身
  _.bindAll = function (obj) {
    var i,
      length = arguments.length,
      key;
    if (length <= 1) throw new Error("bindAll must be passed function names");
    for (i = 1; i < length; i++) {
      key = arguments[i];
      obj[key] = _.bind(obj[key], obj);
    }
    return obj;
  };

  // Memoize an expensive function by storing its results.
  // 缓存
  _.memoize = function (func, hasher) {
    var memoize = function (key) {
      var cache = memoize.cache;
      var address = "" + (hasher ? hasher.apply(this, arguments) : key);
      if (!_.has(cache, address)) cache[address] = func.apply(this, arguments);
      return cache[address];
    };
    memoize.cache = {};
    return memoize;
  };

  // Delays a function for the given number of milliseconds, and then calls
  // it with the arguments supplied.
  // 延迟执行
  _.delay = function (func, wait) {
    var args = slice.call(arguments, 2);
    return setTimeout(function () {
      return func.apply(null, args);
    }, wait);
  };

  // Defers a function, scheduling it to run after the current call stack has
  // cleared.
  // 延迟执行一个函数
  _.defer = _.partial(_.delay, _, 1);

  // Returns a function, that, when invoked, will only be triggered at most once
  // during a given window of time. Normally, the throttled function will run
  // as much as it can, without ever going more than once per `wait` duration;
  // but if you'd like to disable the execution on the leading edge, pass
  // `{leading: false}`. To disable execution on the trailing edge, ditto.
  // 节流方法
  _.throttle = function (func, wait, options) {
    var context, args, result;
    var timeout = null;
    var previous = 0;
    if (!options) options = {};
    var later = function () {
      previous = options.leading === false ? 0 : _.now();
      timeout = null;
      result = func.apply(context, args);
      if (!timeout) context = args = null;
    };
    return function () {
      var now = _.now();
      if (!previous && options.leading === false) previous = now;
      var remaining = wait - (now - previous);
      context = this;
      args = arguments;
      // remaining > wait这个情况几乎不存在，只有可能是在代码执行期间，系统时间被修改改
      if (remaining <= 0 || remaining > wait) {
        if (timeout) {
          clearTimeout(timeout);
          timeout = null;
        }
        previous = now;
        result = func.apply(context, args);
        if (!timeout) context = args = null;
      } else if (!timeout && options.trailing !== false) {
        // 注意这个 else if !timeout 判断，这个避免了后续的多次点击
        timeout = setTimeout(later, remaining);
      }
      return result;
    };
  };

  // Returns a function, that, as long as it continues to be invoked, will not
  // be triggered. The function will be called after it stops being called for
  // N milliseconds. If `immediate` is passed, trigger the function on the
  // leading edge, instead of the trailing.
  // 防抖函数，多次触发只执行一次
  // @param immediate 是否立即执行
  _.debounce = function (func, wait, immediate) {
    var timeout, args, context, timestamp, result;

    var later = function () {
      var last = _.now() - timestamp;

      if (last < wait && last >= 0) {
        timeout = setTimeout(later, wait - last);
      } else {
        timeout = null;
        if (!immediate) {
          result = func.apply(context, args);
          if (!timeout) context = args = null;
        }
      }
    };

    return function () {
      context = this;
      args = arguments;
      timestamp = _.now();
      var callNow = immediate && !timeout;
      if (!timeout) timeout = setTimeout(later, wait);
      if (callNow) {
        result = func.apply(context, args);
        context = args = null;
      }

      return result;
    };
  };

  // Returns the first function passed as an argument to the second,
  // allowing you to adjust arguments, run code before and after, and
  // conditionally execute the original function.
  _.wrap = function (func, wrapper) {
    return _.partial(wrapper, func);
  };

  // Returns a negated version of the passed-in predicate.
  // 反转传入的 predicate 结果
  _.negate = function (predicate) {
    return function () {
      return !predicate.apply(this, arguments);
    };
  };

  // Returns a function that is the composition of a list of functions, each
  // consuming the return value of the function that follows.
  // 函数组合
  _.compose = function () {
    var args = arguments;
    var start = args.length - 1;
    return function () {
      var i = start;
      var result = args[start].apply(this, arguments);
      while (i--) result = args[i].call(this, result);
      return result;
    };
  };

  // Returns a function that will only be executed on and after the Nth call.
  // 当结果函数被执行 times - 1 次以后再执行才执行 func
  _.after = function (times, func) {
    return function () {
      if (--times < 1) {
        return func.apply(this, arguments);
      }
    };
  };

  // Returns a function that will only be executed up to (but not including) the Nth call.
  // 存储 times 次执行结果
  _.before = function (times, func) {
    var memo;
    return function () {
      if (--times > 0) {
        memo = func.apply(this, arguments);
      }
      if (times <= 1) func = null;
      return memo;
    };
  };

  // Returns a function that will be executed at most one time, no matter how
  // often you call it. Useful for lazy initialization.
  // 只执行一次
  _.once = _.partial(_.before, 2);

  // Object Functions
  // ----------------

  // Keys in IE < 9 that won't be iterated by `for key in ...` and thus missed.
  var hasEnumBug = !{ toString: null }.propertyIsEnumerable("toString");
  var nonEnumerableProps = [
    "valueOf",
    "isPrototypeOf",
    "toString",
    "propertyIsEnumerable",
    "hasOwnProperty",
    "toLocaleString",
  ];

  function collectNonEnumProps(obj, keys) {
    var nonEnumIdx = nonEnumerableProps.length;
    var constructor = obj.constructor;
    var proto =
      (_.isFunction(constructor) && constructor.prototype) || ObjProto;

    // Constructor is a special case.
    var prop = "constructor";
    if (_.has(obj, prop) && !_.contains(keys, prop)) keys.push(prop);

    while (nonEnumIdx--) {
      prop = nonEnumerableProps[nonEnumIdx];
      if (prop in obj && obj[prop] !== proto[prop] && !_.contains(keys, prop)) {
        keys.push(prop);
      }
    }
  }

  // es5 Object.keys() 的垫片，获取一个对象所有的**自有keys**
  // 注意，使用var in 遍历对象，也会拿到继承自原型的属性
  _.keys = function (obj) {
    if (!_.isObject(obj)) return [];
    if (nativeKeys) return nativeKeys(obj);
    var keys = [];
    for (var key in obj) if (_.has(obj, key)) keys.push(key);
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
  };

  // Retrieve all the property names of an object.
  // 拿到所有的 key，包含 prototype 上的
  _.allKeys = function (obj) {
    if (!_.isObject(obj)) return [];
    var keys = [];
    for (var key in obj) keys.push(key);
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
  };

  // Retrieve the values of an object's properties.
  // 检索对象，获取所有的 value 值并以数组形式返回
  _.values = function (obj) {
    var keys = _.keys(obj);
    var length = keys.length;
    var values = Array(length);
    for (var i = 0; i < length; i++) {
      values[i] = obj[keys[i]];
    }
    return values;
  };

  // Returns the results of applying the iteratee to each element of the object
  // In contrast to _.map it returns an object
  // object 根据条件进行值映射
  _.mapObject = function (obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    var keys = _.keys(obj),
      length = keys.length,
      results = {},
      currentKey;
    for (var index = 0; index < length; index++) {
      currentKey = keys[index];
      results[currentKey] = iteratee(obj[currentKey], currentKey, obj);
    }
    return results;
  };

  // Convert an object into a list of `[key, value]` pairs.
  // 将对象的 key:value 转化成 [[key, value], [key, value], ...]形式
  _.pairs = function (obj) {
    var keys = _.keys(obj);
    var length = keys.length;
    var pairs = Array(length);
    for (var i = 0; i < length; i++) {
      pairs[i] = [keys[i], obj[keys[i]]];
    }
    return pairs;
  };

  // Invert the keys and values of an object. The values must be serializable.
  // 反转value 和 key
  _.invert = function (obj) {
    var result = {};
    var keys = _.keys(obj);
    for (var i = 0, length = keys.length; i < length; i++) {
      result[obj[keys[i]]] = keys[i];
    }
    return result;
  };

  // Return a sorted list of the function names available on the object.
  // Aliased as `methods`
  // 获取一个对象所有的方法
  _.functions = _.methods = function (obj) {
    var names = [];
    for (var key in obj) {
      if (_.isFunction(obj[key])) names.push(key);
    }
    return names.sort();
  };

  // Extend a given object with all the properties in passed-in object(s).
  // 对象合并
  _.extend = createAssigner(_.allKeys);

  // Assigns a given object with all the own properties in the passed-in object(s)
  // (https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object/assign)
  // 对象合并
  _.extendOwn = _.assign = createAssigner(_.keys);

  // 通过 _.keys拿到所有keys，然后for循环遍历找到符合 predicate 要求的
  _.findKey = function (obj, predicate, context) {
    predicate = cb(predicate, context);
    var keys = _.keys(obj),
      key;
    for (var i = 0, length = keys.length; i < length; i++) {
      key = keys[i];
      if (predicate(obj[key], key, obj)) return key;
    }
  };

  // Return a copy of the object only containing the whitelisted properties.
  // 从对象里找出需要的 key:value
  _.pick = function (object, oiteratee, context) {
    var result = {},
      obj = object,
      iteratee,
      keys;
    if (obj == null) return result;
    if (_.isFunction(oiteratee)) {
      keys = _.allKeys(obj);
      iteratee = optimizeCb(oiteratee, context);
    } else {
      keys = flatten(arguments, false, false, 1);
      iteratee = function (value, key, obj) {
        return key in obj;
      };
      obj = Object(obj);
    }
    for (var i = 0, length = keys.length; i < length; i++) {
      var key = keys[i];
      var value = obj[key];
      if (iteratee(value, key, obj)) result[key] = value;
    }
    return result;
  };

  // Return a copy of the object without the blacklisted properties.
  _.omit = function (obj, iteratee, context) {
    if (_.isFunction(iteratee)) {
      iteratee = _.negate(iteratee);
    } else {
      var keys = _.map(flatten(arguments, false, false, 1), String);
      iteratee = function (value, key) {
        return !_.contains(keys, key);
      };
    }
    return _.pick(obj, iteratee, context);
  };

  // Fill in a given object with default properties.
  _.defaults = createAssigner(_.allKeys, true);

  // Creates an object that inherits from the given prototype object.
  // If additional properties are provided then they will be added to the
  // created object.
  _.create = function (prototype, props) {
    var result = baseCreate(prototype);
    if (props) _.extendOwn(result, props);
    return result;
  };

  // Create a (shallow-cloned) duplicate of an object.
  // 浅克隆
  _.clone = function (obj) {
    if (!_.isObject(obj)) return obj;
    return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
  };

  // Invokes interceptor with the obj, and then returns obj.
  // The primary purpose of this method is to "tap into" a method chain, in
  // order to perform operations on intermediate results within the chain.
  _.tap = function (obj, interceptor) {
    interceptor(obj);
    return obj;

  // 真值判断
  // 判断 attrs 的 key:value 在 object中也全都有，值也都存在，是object的"子集"
  _.isMatch = function (object, attrs) {
    var keys = _.keys(attrs),
      length = keys.length;
    if (object == null) return !length;
    var obj = Object(object);
    for (var i = 0; i < length; i++) {
      var key = keys[i];
      if (attrs[key] !== obj[key] || !(key in obj)) return false;
    }
    return true;
  };

  // Internal recursive comparison function for `isEqual`.
  // 比较是否相等
  var eq = function (a, b, aStack, bStack) {
    // Identical objects are equal. `0 === -0`, but they aren't identical.
    // See the [Harmony `egal` proposal](http://wiki.ecmascript.org/doku.php?id=harmony:egal).
    if (a === b) return a !== 0 || 1 / a === 1 / b;
    // A strict comparison is necessary because `null == undefined`.
    if (a == null || b == null) return a === b;
    // Unwrap any wrapped objects.
    if (a instanceof _) a = a._wrapped;
    if (b instanceof _) b = b._wrapped;
    // Compare `[[Class]]` names.
    var className = toString.call(a);
    if (className !== toString.call(b)) return false;
    switch (className) {
      // Strings, numbers, regular expressions, dates, and booleans are compared by value.
      case "[object RegExp]":
      // RegExps are coerced to strings for comparison (Note: '' + /a/i === '/a/i')
      case "[object String]":
        // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
        // equivalent to `new String("5")`.
        return "" + a === "" + b;
      case "[object Number]":
        // `NaN`s are equivalent, but non-reflexive.
        // Object(NaN) is equivalent to NaN
        if (+a !== +a) return +b !== +b;
        // An `egal` comparison is performed for other numeric values.
        return +a === 0 ? 1 / +a === 1 / b : +a === +b;
      case "[object Date]":
      case "[object Boolean]":
        // Coerce dates and booleans to numeric primitive values. Dates are compared by their
        // millisecond representations. Note that invalid dates with millisecond representations
        // of `NaN` are not equivalent.
        return +a === +b;
    }

    var areArrays = className === "[object Array]";
    if (!areArrays) {
      if (typeof a != "object" || typeof b != "object") return false;

      // Objects with different constructors are not equivalent, but `Object`s or `Array`s
      // from different frames are.
      var aCtor = a.constructor,
        bCtor = b.constructor;
      if (
        aCtor !== bCtor &&
        !(
          _.isFunction(aCtor) &&
          aCtor instanceof aCtor &&
          _.isFunction(bCtor) &&
          bCtor instanceof bCtor
        ) &&
        "constructor" in a &&
        "constructor" in b
      ) {
        return false;
      }
    }
    // Assume equality for cyclic structures. The algorithm for detecting cyclic
    // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.

    // Initializing stack of traversed objects.
    // It's done here since we only need them for objects and arrays comparison.
    aStack = aStack || [];
    bStack = bStack || [];
    var length = aStack.length;
    while (length--) {
      // Linear search. Performance is inversely proportional to the number of
      // unique nested structures.
      if (aStack[length] === a) return bStack[length] === b;
    }

    // Add the first object to the stack of traversed objects.
    aStack.push(a);
    bStack.push(b);

    // Recursively compare objects and arrays.
    if (areArrays) {
      // Compare array lengths to determine if a deep comparison is necessary.
      length = a.length;
      if (length !== b.length) return false;
      // Deep compare the contents, ignoring non-numeric properties.
      while (length--) {
        if (!eq(a[length], b[length], aStack, bStack)) return false;
      }
    } else {
      // Deep compare objects.
      var keys = _.keys(a),
        key;
      length = keys.length;
      // Ensure that both objects contain the same number of properties before comparing deep equality.
      if (_.keys(b).length !== length) return false;
      while (length--) {
        // Deep compare each member
        key = keys[length];
        if (!(_.has(b, key) && eq(a[key], b[key], aStack, bStack)))
          return false;
      }
    }
    // Remove the first object from the stack of traversed objects.
    aStack.pop();
    bStack.pop();
    return true;
  };

  // Perform a deep comparison to check if two objects are equal.
  _.isEqual = function (a, b) {
    return eq(a, b);
  };

  // Is a given array, string, or object empty?
  // An "empty" object has no enumerable own-properties.
  // 判断是否是空数组，空字符串或者空对象
  _.isEmpty = function (obj) {
    if (obj == null) return true;
    if (
      isArrayLike(obj) &&
      (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))
    )
      return obj.length === 0;
    return _.keys(obj).length === 0;
  };

  // Is a given value a DOM element?
  // 判断是否是 dom 元素
  _.isElement = function (obj) {
    return !!(obj && obj.nodeType === 1);
  };

  // Is a given value an array?
  // Delegates to ECMA5's native Array.isArray
  // 判断是否是数组
  _.isArray =
    nativeIsArray ||
    function (obj) {
      return toString.call(obj) === "[object Array]";
    };

  // 判断是否是对象
  // 函数和{}形式符合条件，其它不行
  // 注意 typeof null === 'object'
  _.isObject = function (obj) {
    var type = typeof obj;
    return type === "function" || (type === "object" && !!obj);
  };

  // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError.
  // 其它的一些类型判断
  _.each(
    ["Arguments", "Function", "String", "Number", "Date", "RegExp", "Error"],
    function (name) {
      _["is" + name] = function (obj) {
        return toString.call(obj) === "[object " + name + "]";
      };
    }
  );

  // Define a fallback version of the method in browsers (ahem, IE < 9), where
  // there isn't any inspectable "Arguments" type.
  if (!_.isArguments(arguments)) {
    _.isArguments = function (obj) {
      return _.has(obj, "callee");
    };
  }

  // Optimize `isFunction` if appropriate. Work around some typeof bugs in old v8,
  // IE 11 (#1621), and in Safari 8 (#1929).
  if (typeof /./ != "function" && typeof Int8Array != "object") {
    _.isFunction = function (obj) {
      return typeof obj == "function" || false;
    };
  }

  // Is a given object a finite number?
  _.isFinite = function (obj) {
    return isFinite(obj) && !isNaN(parseFloat(obj));
  };

  // Is the given value `NaN`? (NaN is the only number which does not equal itself).
  // 判断是否是 NaN
  _.isNaN = function (obj) {
    return _.isNumber(obj) && obj !== +obj;
  };

  // Is a given value a boolean?
  // 判断是否是布尔值
  _.isBoolean = function (obj) {
    return (
      obj === true || obj === false || toString.call(obj) === "[object Boolean]"
    );
  };

  // Is a given value equal to null?
  // 判断 null
  _.isNull = function (obj) {
    return obj === null;
  };

  // Is a given variable undefined?
  // 判断是否是 undefined
  _.isUndefined = function (obj) {
    return obj === void 0;
  };

  // 判断是否是自有属性
  _.has = function (obj, key) {
    return obj != null && hasOwnProperty.call(obj, key);
  };

  // Utility Functions
  // -----------------

  // Run Underscore.js in *noConflict* mode, returning the `_` variable to its
  // previous owner. Returns a reference to the Underscore object.
  // 防冲突
  _.noConflict = function () {
    root._ = previousUnderscore;
    return this;
  };

  // Keep the identity function around for default iteratees.
  // 返回值本身，用于内置函数 cb
  _.identity = function (value) {
    return value;
  };

  // Predicate-generating functions. Often useful outside of Underscore.
  // 用函数的方式创建一个常量，只能访问不能修改
  _.constant = function (value) {
    return function () {
      return value;
    };
  };

  // 空函数
  _.noop = function () {};

  // 返回一个函数，获取它key对应的value
  _.property = property;

  // Generates a function for a given object that returns a given property.
  // 返回一个函数，获取 obj 对应 key
  _.propertyOf = function (obj) {
    return obj == null
      ? function () {}
      : function (key) {
          return obj[key];
        };
  };

  // 柯里化，判断一个给定的对象 attrs 的 key:value 是否都在在后面 obj 中
  _.matcher = _.matches = function (attrs) {
    attrs = _.extendOwn({}, attrs);
    return function (obj) {
      return _.isMatch(obj, attrs);
    };
  };

  // Run a function **n** times.
  // 操作一个函数 n 次，返回结果数组
  _.times = function (n, iteratee, context) {
    var accum = Array(Math.max(0, n));
    iteratee = optimizeCb(iteratee, context, 1);
    for (var i = 0; i < n; i++) accum[i] = iteratee(i);
    return accum;
  };

  // Return a random integer between min and max (inclusive).
  // 返回 min 到 max 的一个随机数
  _.random = function (min, max) {
    if (max == null) {
      max = min;
      min = 0;
    }
    return min + Math.floor(Math.random() * (max - min + 1));
  };

  // A (possibly faster) way to get the current timestamp as an integer.
  // 获取当前时间戳
  _.now =
    Date.now ||
    function () {
      return new Date().getTime();
    };

  // List of HTML entities for escaping.
  // html 里需要用到的转义符
  var escapeMap = {
    "&": "&amp;",
    "<": "&lt;",
    ">": "&gt;",
    '"': "&quot;",
    "'": "&#x27;",
    "`": "&#x60;",
  };
  var unescapeMap = _.invert(escapeMap);

  // Functions for escaping and unescaping strings to/from HTML interpolation.
  var createEscaper = function (map) {
    var escaper = function (match) {
      return map[match];
    };
    // Regexes for identifying a key that needs to be escaped
    var source = "(?:" + _.keys(map).join("|") + ")";
    var testRegexp = RegExp(source);
    var replaceRegexp = RegExp(source, "g");
    return function (string) {
      string = string == null ? "" : "" + string;
      return testRegexp.test(string)
        ? string.replace(replaceRegexp, escaper)
        : string;
    };
  };
  _.escape = createEscaper(escapeMap);
  _.unescape = createEscaper(unescapeMap);

  // If the value of the named `property` is a function then invoke it with the
  // `object` as context; otherwise, return it.
  _.result = function (object, property, fallback) {
    var value = object == null ? void 0 : object[property];
    if (value === void 0) {
      value = fallback;
    }
    return _.isFunction(value) ? value.call(object) : value;
  };

  // Generate a unique integer id (unique within the entire client session).
  // Useful for temporary DOM ids.
  // 生成一个临时唯一的id
  var idCounter = 0;
  _.uniqueId = function (prefix) {
    var id = ++idCounter + "";
    return prefix ? prefix + id : id;
  };

  // By default, Underscore uses ERB-style template delimiters, change the
  // following template settings to use alternative delimiters.
  _.templateSettings = {
    evaluate: /<%([\s\S]+?)%>/g,
    interpolate: /<%=([\s\S]+?)%>/g,
    escape: /<%-([\s\S]+?)%>/g,
  };

  // When customizing `templateSettings`, if you don't want to define an
  // interpolation, evaluation or escaping regex, we need one that is
  // guaranteed not to match.
  var noMatch = /(.)^/;

  // Certain characters need to be escaped so that they can be put into a
  // string literal.
  var escapes = {
    "'": "'",
    "\\": "\\",
    "\r": "r",
    "\n": "n",
    "\u2028": "u2028",
    "\u2029": "u2029",
  };

  var escaper = /\\|'|\r|\n|\u2028|\u2029/g;

  var escapeChar = function (match) {
    return "\\" + escapes[match];
  };

  // JavaScript micro-templating, similar to John Resig's implementation.
  // Underscore templating handles arbitrary delimiters, preserves whitespace,
  // and correctly escapes quotes within interpolated code.
  // NB: `oldSettings` only exists for backwards compatibility.
  // 模板引擎基本看完
  _.template = function (text, settings, oldSettings) {
    if (!settings && oldSettings) settings = oldSettings;
    settings = _.defaults({}, settings, _.templateSettings);

    // Combine delimiters into one regular expression via alternation.
    var matcher = RegExp(
      [
        (settings.escape || noMatch).source,
        (settings.interpolate || noMatch).source,
        (settings.evaluate || noMatch).source,
      ].join("|") + "|$",
      "g"
    );

    // Compile the template source, escaping string literals appropriately.
    var index = 0;
    var source = "__p+='";
    text.replace(
      matcher,
      function (match, escape, interpolate, evaluate, offset) {
        source += text.slice(index, offset).replace(escaper, escapeChar);
        index = offset + match.length;

        if (escape) {
          source += "'+\n((__t=(" + escape + "))==null?'':_.escape(__t))+\n'";
        } else if (interpolate) {
          source += "'+\n((__t=(" + interpolate + "))==null?'':__t)+\n'";
        } else if (evaluate) {
          source += "';\n" + evaluate + "\n__p+='";
        }

        // Adobe VMs need the match returned to produce the correct offest.
        return match;
      }
    );
    source += "';\n";

    // If a variable is not specified, place data values in local scope.
    if (!settings.variable) source = "with(obj||{}){\n" + source + "}\n";

    source =
      "var __t,__p='',__j=Array.prototype.join," +
      "print=function(){__p+=__j.call(arguments,'');};\n" +
      source +
      "return __p;\n";

    try {
      var render = new Function(settings.variable || "obj", "_", source);
    } catch (e) {
      e.source = source;
      throw e;
    }

    var template = function (data) {
      return render.call(this, data, _);
    };

    // Provide the compiled source as a convenience for precompilation.
    var argument = settings.variable || "obj";
    template.source = "function(" + argument + "){\n" + source + "}";

    return template;
  };

  // Add a "chain" function. Start chaining a wrapped Underscore object.
  // 链式调用
  _.chain = function (obj) {
    var instance = _(obj);
    instance._chain = true;
    return instance;
  };

  // OOP
  // ---------------
  // If Underscore is called as a function, it returns a wrapped object that
  // can be used OO-style. This wrapper holds altered versions of all the
  // underscore functions. Wrapped objects may be chained.

  // Helper function to continue chaining intermediate results.
  var result = function (instance, obj) {
    return instance._chain ? _(obj).chain() : obj;
  };

  // Add your own custom functions to the Underscore object.
  // 混入自定义方法
  _.mixin = function (obj) {
    _.each(_.functions(obj), function (name) {
      var func = (_[name] = obj[name]);
      _.prototype[name] = function () {
        var args = [this._wrapped];
        push.apply(args, arguments);
        return result(this, func.apply(_, args));
      };
    });
  };

  // Add all of the Underscore functions to the wrapper object.
  _.mixin(_);

  // Add all mutator Array functions to the wrapper.
  _.each(
    ["pop", "push", "reverse", "shift", "sort", "splice", "unshift"],
    function (name) {
      var method = ArrayProto[name];
      _.prototype[name] = function () {
        var obj = this._wrapped;
        method.apply(obj, arguments);
        if ((name === "shift" || name === "splice") && obj.length === 0)
          delete obj[0];
        return result(this, obj);
      };
    }
  );

  // Add all accessor Array functions to the wrapper.
  _.each(["concat", "join", "slice"], function (name) {
    var method = ArrayProto[name];
    _.prototype[name] = function () {
      return result(this, method.apply(this._wrapped, arguments));
    };
  });

  // Extracts the result from a wrapped and chained object.
  _.prototype.value = function () {
    return this._wrapped;
  };

  // Provide unwrapping proxy for some methods used in engine operations
  // such as arithmetic and JSON stringification.
  _.prototype.valueOf = _.prototype.toJSON = _.prototype.value;

  _.prototype.toString = function () {
    return "" + this._wrapped;
  };

  // AMD registration happens at the end for compatibility with AMD loaders
  // that may not enforce next-turn semantics on modules. Even though general
  // practice for AMD registration is to be anonymous, underscore registers
  // as a named module because, like jQuery, it is a base library that is
  // popular enough to be bundled in a third party lib, but not be part of
  // an AMD load request. Those cases could generate an error when an
  // anonymous define() is called outside of a loader request.
  if (typeof define === "function" && define.amd) {
    define("underscore", [], function () {
      return _;
    });
  }
}.call(this));
